JP2002048693A - Evaluation method for high-fatigue-strength material in high-strength steel and high-fatigue-strength material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an evaluation method, for a high-fatigue-strength material in high-strength steel, in which a relationship between a defect size and a fatigue strength is taken into consideration and to provide the high-fatigue-strength material. SOLUTION: In steel whose tensile strength σB (unit: MPA) is known and whose Vickers hardness Hv is known, a fracture starting point is set only on its surface. Then, its defect area is measured. When the defect size area is at 45.8/σB2 or 4.47/Hv2, the design guideline of the material whose fatigue limit σw (unit: MPa) reaches σw>=0.53sB or σw>=1.6 Hv is given.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating a high fatigue strength material in high strength steel and a high fatigue strength material produced by the evaluation method. More specifically, the invention of this application relates to a high-strength steel useful for material design and manufacture of gigacycle fatigue strength for achieving lighter weight, smaller size, and higher performance of mechanical structures and automobile parts. The present invention relates to a method for evaluating high fatigue strength materials in steel and high fatigue strength materials.

[0002]

2. Description of the Related Art Conventionally, in order to reduce the weight, size, and performance of mechanical structures and automobile parts, high-strength steels having high fatigue strength on the order of gigacycles have been required. ing. However, especially in the gigacycle range of 1200 MPa super-grade high-strength steel, fatigue strength is reduced due to internal destruction starting from inclusions and structural cracks, and high-strength steel with high fatigue strength must be manufactured. Is not technically easy. Furthermore, due to the complicated mechanism of internal fracture, there is almost no known material design method for producing high-strength steel having higher fatigue strength.

[0003] For example, as one of material design methods for manufacturing high-strength steel, attention is paid to the defect size and properties of inclusions, and the defect size of inclusions is calculated from a correlation equation between the inclusion size and the fatigue limit. There is known a method of measuring the fatigue limit by measuring.

[0004] However, this method merely simplifies a complicated internal failure mechanism and grasps an approximate fatigue limit. Therefore, this method is designed to produce a high-strength steel with higher fatigue strength. It does not give a law.

In such a situation, recently,
As one clue to elucidating the complex internal fracture mechanism, an ODA (Optically Dark Area), which is thought to be crack propagation due to hydrogen around inclusions, has been observed. Attempts to elucidate have been made. The discovery of the ODA defect region may contribute to the design of high-strength steels with higher fatigue strength in the future, but has yet to give specific design guidelines. Absent.

[0006] The invention of this application has been made in view of the above circumstances, and a method for evaluating a high fatigue strength material in a high strength steel in consideration of the relationship between the ODA defect size and the fatigue strength. And high fatigue strength materials.

[0007]

SUMMARY OF THE INVENTION The claimed invention is as to solve the above problems, the first, the tensile strength sigma _B
(Unit: MPa) In a steel whose Vickers hardness Hv is known, when the fracture origin is only the surface, the √area (unit: m) of the defect contained in the steel is 45.8 / σ.
_{When B} ² or 4.47 / Hv ² or less, the fatigue limit σ
_w (in MPa), σ _w ≧ 0.5σ _B or σ _w
Provided is a method for evaluating a material having a high fatigue strength in a high-strength steel, which provides a material design guideline for achieving ≧ 1.6 Hv.

[0008] Secondly, the invention of this application relates to a tensile strength σ.
_B (unit: MPa) In a steel whose Vickers hardness Hv is known, when the fracture origin is inside, the defect area area of the inclusion is measured, and the fatigue limit σ _w (unit: MPa)
Provide a high fatigue strength material in a high strength steel characterized by providing a material design guideline that achieves σ _w ≧ 3.38 (area _i ) ^−1/4. A method that enables the evaluation of some high fatigue strength materials,
If tried to tissue uniform (size reduction of heterogeneous tissue) or grain refiner (block width decreases), the maximum inhomogeneous structure area area _{ma x,} a _m was measured, the maximum inhomogeneous structure dimensions} area _{max, m} (√area _{max, m} is in μm)
Is a straight line area _{max, m} = 0 and √area
_{max, m} = 0.9403y + 4.571 (inspection reference area S
₀ = 6.2 × 10 ⁻⁹ m ² ), or the extreme value distribution of the maximum block width d _max (the unit of d _max is μm) has straight lines d _max = 0 and d _max = 0. 217y + 0.
701 (y is a standardized variable, inspection standard area 1 × 10 ⁻¹⁰ m
² ) To provide a method for evaluating a high-strength tissue characterized by being in an area surrounded by ² ).

Fourth, the invention of the present application is
According to the evaluation method of the third invention, the first or the second
Steel is manufactured in accordance with the material design guidelines given by the invention.
A method for producing high fatigue strength materials characterized by
5th, 2 × 10^-6 Steel in high vacuum over Pa
By performing a tempering heat treatment, the method of the first invention can be performed.
Manufacturing steel in accordance with the given material design guidelines.
And a method for producing a high fatigue strength material.

Further, the invention of this application is, sixthly, manufactured under the third invention in accordance with the material design guidelines given by the first or second invention evaluation method. Provide high fatigue strength material.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The invention of this application has the features as described above, and embodiments thereof will be described below.

The invention of this application focuses on the correlation between the defect size of inclusions including the ODA defect size and the fatigue strength.
By quantifying the defect size, mainly in the three cases where the fracture origin is only the surface, a) when the fracture origin is internal, and c) when the structure is homogenized or the crystal grains are refined, There is a great feature in calculating the fatigue strength.

Therefore, the invention of this application clarifies the correlation between the fatigue strength and the size of the inclusions, so that it is possible to provide inclusions of an accurate size, and even if the mechanical strength is the same. Accordingly, it is possible to manufacture a high-strength steel having a higher fatigue strength, and it is possible to improve reliability in fatigue design of the high-strength steel.

In the present invention, the defect size of the inclusion refers to a defect area route to a plane perpendicular to the load direction of the inclusion, and is a unit of length in dimension. The maximum non-uniform structure defect size indicates a route of the maximum value of the defect area to a plane perpendicular to the load direction of the inclusion non-uniform structure, and is a unit of length on a dimension.

Further, the present invention also makes it possible to provide a high-strength steel manufactured by the design method. Specifically, tensile strength σ _B (unit is MPa), Vickers hardness Hv, fatigue limit σ _w (unit is MPa) σ _w ≧
High strength steels having a fatigue strength of as much as 0.5 σ _B or σ _W ≧ 1.6 Hv ² are also provided. There is no high-strength steel having such fatigue strength to date.

In a steel having a known tensile strength σ _B (unit is MPa) and Vickers hardness Hv, first,
A) A method of obtaining the fatigue limit σ _W for a steel in which the fracture origin is only the surface (surface fracture occurs prior to internal fracture) will be described.

In this case, since the fracture origin is only the surface, it is assumed that the fatigue limit σ _wsurface of the surface fracture is 0.5 σ _B or a high-strength steel which does not generate ODA exceeding 1.6 Hv. As a special case of the defect √ area (unit: m), the maximum value (maximum defect) (√ area) _max (unit: m) of the defect area in which surface rupture occurs prior to internal destruction is
Using the fatigue limit σ _wsurface of surface fracture,

[0018]

(Equation 1)

## EQU1 ## Here, ΔK _th is a lower limit value of a stress intensity factor range in which a crack propagates, and its general value is about 3 MPam ^1/2 . Therefore, the maximum defect size (√area) _max is the tensile strength σ _B (unit is MP
Using a) and Vickers hardness Hv,

[0020]

(Equation 2)

## EQU1 ## From this equation (2), when the fracture origin is only the surface, the defect size √area is 45.8.
/ Σ _B ² or 4.47 / Hv ² or less, the fatigue limit σ _w is such that σ _w is ≧ 0.5σ _B or σ _w ≧ 1.6Hv ²
Can provide a material design guide that can produce high-strength steel.

Next, a) how to determine the fatigue limit σ _w when the fracture starting point is inside will be described. Since the fatigue limit at the internal fracture starting point is prioritized, if (defect size of inclusions) is area _i (√area _i is in m), the fatigue limit σ _w becomes

[0023]

(Equation 3)

## EQU1 ## Here, assuming that ΔK _th = 3 MPam ^1/2 , the fatigue limit σ _w becomes

[0025]

(Equation 4)

## EQU1 ## That is, in the case of steel in which ODA does not occur and the size of inclusions is not restricted, the defect area of the inclusions area _i
Is measured, the fatigue limit σ _w is 3.38 (area _i )
Guidance on material design that will be more than ^-1/4 is given.

Next, (c) As a method for evaluating a high-strength structure that enables the above-mentioned a) and a), a case where the structure is uniform (reduction in the size of a non-uniform structure) or finer (the block width is reduced) will be described. I do.

That is, in this case, the maximum non-uniform tissue defect area area _{max, m} is measured, and the maximum non-uniform tissue defect size √
The extreme value distribution of area _{max, m} is a straight line, √area _{max, m} =
0.9403y + 4.571 and} area _max, in the area surrounded by the m = 0, in regions where extreme value distribution of the maximum block width d _m is surrounded by the straight line d _max = 0.217y + 0.701 and d _max = 0 It is necessary to be. In this case, inspection reference area for determining the maximum inhomogeneous structure dimensions} area _m is ^{_{S 0 = 6.2 × 10 -9 m}} 2 about desirably,
The inspection reference area of the maximum block width d _max is S ₀ = 1 × 10
^{About -10} m ² is desirable.

Hereinafter, the present invention will be described in detail with reference to Examples.

[0030]

【Example】Example 1 A Sample preparation and pretreatment  To demonstrate the accuracy of the evaluation method of the invention of this application,
Actually, using the steel SUP12 and the valve spring steel SWOSC-V
Was tested. First, preparation of the sample and preparation
And the mechanical properties of the samples were investigated.

Spring steel SUP12 and valve spring steel SWOSC-
The chemical composition of V is as shown in Table 1. Depending on the type of heat, four types of spring steel SUP12 (heat A, B
2, C1, D1,) and two types (E2, F) of valve spring steel SWOSC-V were prepared. The shape and defect size of these steels, heat treatment conditions, tensile strength σ _B , and Vickers hardness Hv are as shown in Table 2, and the spring steel SUP
Regarding the heat D1 of No. 12, two types having different heat treatment conditions, that is, a normal quenched and tempered material (QT material) and an improved aus foam material (AF material) were prepared.

[0032]

[Table 1]

[0033]

[Table 2]

For these six types of steel in total, first, an extreme value statistical graph was prepared in order to obtain the maximum defect size √area _{max , m} . In the case of this embodiment, the defect size of the largest inclusion guarantees the case of only surface destruction without internal destruction.

The extreme value statistical graph shows the inspection reference area S _0.
= Maximum defect size at 0.482 mm ² √area
_{The maximum and m} were obtained, and this procedure was performed 20 times at different places to obtain the distribution line of the maximum inclusion. Based on this distribution straight line, the minimum defect size of the fatigue test piece (πr ² = π × 6 ² = 28.3)
mm ² ), and r is the maximum defect size √are existing in the radius).
a _{max, m} was determined and is shown in Table 3.

[0036]

[Table 3]

According to Table 3, √ of SUP12 steel A heat
The inclusions showing area _{max, m} = 15 μm are Al ₂ O ₃ inclusions, but the other four SUP12 steels (B
2, C1, D1 heat) and two types of SWOSC-V steel (E2, F heat) are Al ₂ O ₃ -based composite inclusions or SiO ₂ -based inclusions, and the latest inclusion softening control is performed. You can see that it is.

1200 MPa which is the object of the present invention
The tensile strength sigma _w and relationships 10 ³ times the fatigue limit of the super-class high-strength steel FIG. 1 (a), the relationship of Vickers hardness Hv and 10 ⁸ times fatigue limit be as shown in FIG. 1 (b) In these figures, the surface fracture of the SUP12 steel AF material {mark and internal fracture}
The mark indicates the internal fracture of the SUP12 steel QT material.
Further, in these figures, the 10 ⁸ times fatigue limit obtained from 11 and 12 to be described later are denoted by the mark and ◇ mark ◆. The average size of the defect size of the inclusions without ODA observed on the fracture surface is 20μ, which is the defect size of internal fracture (marked by ◇).
m was used. These figures also show low strength steel (carbon steel,
+ Low alloy steel, spring steel)
The mark and the case of internal destruction are marked with x. In the result of the internal fracture of the low-strength steel indicated by the mark x in this figure, the fracture origin is A
It was l ₂ O ₃ inclusions. Σ _w = 0.5 for low strength steel
Although a relationship of × σ _B is recognized, there is no correlation between the tensile strength and the fatigue limit in the high-strength steel of the improved aus foam material (AF material) of SUP12 steel and the normal quenched and tempered material of SUP12 steel (QT material). .

From FIGS. 1 (a) and 1 (b), the inside of the AF material
Σ_w= 0.5 or σ_B, Σ_w=
It was found that the value exceeded 1.6 Hv.
It can be understood that steel with high fatigue strength can be designed by the method.B. Evaluation method considering internal fracture origin  Next, recently, a SEM photograph around the internal fracture origin is shown.
Observed by SEM photograph around the inclusion as shown in Fig. 2 (a)
Of irregularities around inclusions (ODA; Opti
cally Dark Area) formed prior to normal fatigue cracking
Is expected to increase the effective defect size of inclusions
This has been taken into account. Ie
As a result of investigating the high-strength steels subjected to the fatigue test,
As shown in Fig. 3, the defect size and the inclusion size
Seki, that is, a correlation of D = 2d was found.

Here, D is the vertical axis, the total defect size √area _h including the defect size due to ODA in the internal fracture starting from the inclusion, d is the horizontal axis, and the fracture origin is the internal In a certain case (in the case where there is no influence of the ODA), the defect size of only the inclusion is 介 在 area _i . FIG. 4 schematically shows this state.

From the above, the fatigue limit σ _w and the defect size √
FIG. 5 shows the relationship between the areas. In FIG.
The 10 ⁸ fatigue limit obtained from FIG. 11 and FIG. 12 described below is indicated by Δ and Δ marks, and the defect size of surface fracture (Δ mark) is the maximum non-uniform structure defect of the D1 heat AF material in Table 3. It was assumed that the dimension √area _{max, m} = 5 μm existed. Here, the defect dimensions} area on the horizontal axis, inclusions originated internal fracture entire defect size} area _h including ODA is, inclusion defects dimensions} area _i of the inclusions in the absence of regulation in the defect size only , And the defect size √area _{max, m} in the case of internal fracture at the structural crack origin. The straight line (a) in the figure showing the correlation is

[0042]

(Equation 5)

Is given by

Here, a straight line (a) when ΔKth = 3 MPam ^1/2 and σ _w representing the fatigue limit of surface fracture.
From the intersection of the straight lines (b) given by ≧ 0.5 × σ _B or σ _w ≧ 1.6Hv, the maximum defect size (√area)
_max is

[0045]

(Equation 6)

Is as follows.

From this equation (6), it can be seen that
Is controlled by limiting the defect size to a value below this lower limit.
The fatigue limit of high-strength steel is 0.5 times or more of tensile strength or Vicca
It can be seen that the hardness is 1.6 times or more of the base hardness.C When there is no regulation on the size of inclusion defects  On the other hand, regarding the internal fracture originating from the inclusion, O
Assuming that the influence of DA is negligible, FIG.
As described above, the defect size is the defect size of the inclusion 介 在 area_iYou
That is, the defect size including the ODA √ area_hHalf of
Σ at the same tensile strength_w= 3.38 (area_i)
^-1/4High strength steel with high fatigue strength expressed by
Understand.Example 2  Next, for SUP12 steel with refined grains with uniform structure,
The high strength steel was designed by applying the evaluation method of the present invention.
Was.

FIG. 6 shows a graph of S
Fig. 3 illustrates a thermomechanical process applied to UP12 steel. That is, 50 mm
The round bar having a diameter was held in an electric furnace at 1200 ° C. for 1 hour, then rolled to a plate thickness of 25 mm, and air-cooled. As the improved ausform treatment, the steel sheet was kept at 845 ° C. for 30 minutes in an electric furnace, air-cooled to 800 ° C., rolled to a sheet thickness of 12 mm in two passes, and water-cooled. As the tempering, it was kept at 430 ° C. for 1 hour in a salt bath and then cooled with water. Vickers hardness is 534.

FIG. 7 is a photograph showing an example of the structure of the improved aus foam material (AF material) and the structure of a normal quenched and tempered material (QT material). Each tissue contained a heterogeneous tissue, and the results of extreme value statistic of the defective area √area were as shown in FIG. The extreme value distribution of the maximum non-uniform structure defect area √area _{max, m} of the AF material is represented by a distribution straight line √area _{max, m} = 0.9403y + 4.571 of the QT material.
And defect size surrounded by √area _{max, m} = 0.

FIG. 9 shows an example of the structure of tempered martensite of the AF material and the QT material. FIG. 10 shows an extreme value statistical result of the representative defect size of this structure as a block width. The extreme value distribution of the maximum value d _max of the block width of the AF material is:
It was in the region surrounded by the distribution linear d _max = 0.217y + 0.701 and d _max = 0 of QT material.

FIG. 11 shows the result of a fatigue test of an AF material having such a structure characteristic. In the case of a surface fracture, it is as shown in FIG. 11, and shows the relationship between the number of fractures Nf and the stress amplitude σa. In the defect shown in FIG.
Those of 8 / σ _B ² or less achieve σ _w ≧ 0.5σ _B corresponding to the first invention of the present application.

Further, the results of the fatigue test of this AF material
In the case of partial destruction, it is as shown in FIG.
The relationship between the number of turns Nf and the stress amplitude σa is shown. This figure
欠 陥 area> 45.8 / σ of the defect shown in FIG._B ^TwoBecomes
Is the defect size of inclusions_iAverage of about 20
μm inclusions cause internal destruction, but around inclusions
ODA does not exist in the second invention of the present application.
Hit σ_w≧ 3.38 (area_i)^-1/4Have achieved
You.Example 2  2 × 10 SUP12 steel manufactured by normal heat treatment^-6P
Heated to 300 ° C. in a high vacuum of at least a. Vickers
The hardness is 518, and the defect size √area is 4.8 / σ.
_B ^TwoAnd σ_w≧ 1.6 Hv was achieved.

[0053]

As described above in detail, according to the invention of this application, there is provided a method of designing a high fatigue strength material for high strength steel in consideration of the relationship between the ODA defect size and the fatigue strength, and the material. To make things possible.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention, in which static strength and 10 ⁸
FIG. 4 is a relationship diagram showing the relationship between the times of fatigue.

FIG. 2 is a view showing an SEM photograph around an internal fracture starting point.

FIG. 3 is a relationship diagram showing a relationship between ODA and a defect size Δarea of an inclusion in the embodiment of the present invention.

FIG. 4 is a schematic head showing a state of internal destruction.

FIG. 5 shows an embodiment of the present invention, in which a defect size ｒｅare
It is a relation diagram showing the relation between a and the fatigue limit.

FIG. 6 is a diagram illustrating a process of thermomechanical treatment.

FIG. 7 is a diagram showing an optical micrograph of a non-uniform structure.

FIG. 8 is a relationship diagram showing an extreme value statistic of an inclusion non-uniform structure defect size in an example of the present invention.

FIG. 9 shows a block diagram of a tempered martensitic structure by AFM.
It is the figure shown as a photograph.

FIG. 10 is a relationship diagram showing an extreme value statistic of the maximum block width according to the embodiment of the present invention.

FIG. 11 is a relationship diagram showing a fatigue test result (with inclusions restricted) of an AF material according to an example of the present invention.

FIG. 12 is a relationship diagram showing a fatigue test result (without inclusion control) of an AF material according to an example of the present invention.

Continuing from the front page (72) Inventor Saburo Matsuoka 1-2-1, Sengen, Tsukuba, Ibaraki Pref., National Institute for Science and Technology (72) Inventor Takayuki Abe 1-2-1, Sengen, Tsukuba, Ibaraki Scientific Technology (72) Inventor, Etsio Takeuchi 1-2-1, Sengen, Tsukuba City, Ibaraki Prefecture, Japan Science and Technology Agency (72) Inventor Kensuke Miyahara 1-2-1, Sengen, Tsukuba City, Ibaraki Prefecture Within the National Institute for Metals Technology, Science and Technology Agency (72) Inventor Kotobuki Hirukawa 1-2-1, Sengen, Tsukuba City, Ibaraki Prefecture Within the National Institute for Metals Technology, Science and Technology Agency (72) Kaneaki Tsuzaki 1-2-2 Sengen, Tsukuba City, Ibaraki Prefecture No. 1 Science and Technology Agency Metal Materials Research Laboratory (72) Inventor Yuji Kimura 1-2-1 Sengen, Tsukuba City, Ibaraki Prefecture Science and Technology Agency Metal Materials Research Laboratory F-term (reference) 2G055 AA03 BA05 BA07 BA14 CA02 CA05 CA06 CA09 CA11 CA22 CA25 CA26 CA27 CA29 EA04 FA01 FA02 2G061 AB05 AC03 BA01 BA15 CA02

Claims (6)

[Claims] 1. In a steel having a known tensile strength σ _B (unit: MPa) and Vickers hardness Hv, when the fracture origin is only the surface, the area of defects included in the steel is
(Unit: m) is 45.8 / σ _B ² or 4.47 / H
When v ² or less, the fatigue limit σ _w (in MPa) is
Evaluation of high fatigue strength material in high strength steel, characterized in that providing design guidelines of the material that sigma _w to achieve ≧ 0.5σ _B or σ _w ≧ 1.6Hv. 2. In a steel having a known tensile strength σ _B (unit: MPa) and Vickers hardness Hv, when a fracture starting point is inside, a defect area area of an inclusion is measured, and a fatigue limit σ _w (unit: Is MPa), but σ _w ≧ 3.38 (area
_i ) An evaluation method for high-fatigue-strength materials in high-strength steels, which provides a material design guide to achieve ^-1/4 . 3. A method for evaluating a material having high fatigue strength according to claim 1 or 2, wherein the structure is maximum when the structure is uniform (reduction in the size of a non-uniform structure) or the structure is refined (reduction in block width). The non-uniform tissue defect area area _{max, m} is measured, and the maximum non-uniform tissue defect area √area _{max, m} (√a
rea _max, the extreme value distribution unit of _m [mu] m), linear √ar
ea _{max, m} = 0 and √area _{max, m} = 0.9403y +
4.571 (inspection reference area S ₀ = 6.2 × 10 ⁻⁹ m ² )
Or the maximum block width d _max
The extreme value distribution (the unit of d _max is μm) is represented by a straight line d _max = 0
And d _max = 0.217y + 0.701 (y is a standardized variable, inspection reference area 1 × 10 ⁻¹⁰ m ² ). 4. The maximum non-uniform structure defect area a when the structure is uniform (reduction in the size of the non-uniform structure) or the structure is refined (the block width is reduced) to enable the evaluation of the high-strength structure.
rea _max, a _m was measured, the maximum inhomogeneous structure area √ar
The extreme value distribution of ea _{max, m} (the unit of √area _{max, m} is μm) is represented by straight lines √area _{max, m} = 0 and √area _{max, m} =
0.9403y + 4.571, or the extreme value distribution of the maximum block width d _max (the unit of d _max is μm) is represented by straight lines d _max = 0 and d _max = 0.217y +
2. A method for evaluating a high-strength tissue according to claim 1, wherein the high-strength tissue is in an area surrounded by 0.701 (y is a normalization variable).
Or a method of manufacturing a high fatigue strength material, wherein steel is manufactured according to the design guidelines given by the evaluation method of 2. 5. The method according to claim 1, wherein the steel is subjected to a heat treatment for tempering the steel in a high vacuum of 2 × 10 ⁻⁶ Pa or more to produce the steel in accordance with the material design guidelines given by the evaluation method. Method for producing a high fatigue strength material. 6. The maximum non-uniform structure defect area a when the structure is made uniform (reduction in the size of the non-uniform structure) or the structure is refined (the block width is reduced) to enable evaluation of the high-strength structure.
rea _max, a _m was measured, the maximum inhomogeneous structure area √ar
The extreme value distribution of ea _{max, m} (the unit of √area _{max, m} is μm) is represented by straight lines √area _{max, m} = 0 and √area _{max, m} =
0.9403y + 4.571, or the extreme value distribution of the maximum block width d _max (the unit of d _max is μm) is represented by straight lines d _max = 0 and d _max = 0.217y +
2. A method for evaluating a high-strength tissue according to claim 1, wherein the high-strength tissue is in an area surrounded by 0.701 (y is a normalization variable).
Or a high fatigue strength material manufactured according to a design guide given by the method for evaluating a high fatigue strength material in the high strength steel of 2.